This invention relates to a film structure having a ferroelectric thin film, an electronic device comprising the film structure, and a recording medium utilizing the film structure. It also relates to a process for preparing a ferroelectric thin film.
The film structure finds use as electronic devices having a ferroelectric film incorporated in a semiconductor device including non-volatile memories, infrared sensors, optical modulators, optical switches, and opto-electronic integrated circuits (OEIC) as well as recording media of the type wherein information is recorded by inducing polarization reversal in ferroelectric material by means of an atomic force microscope (AFM) probe etc.
Electronic devices have been elaborated which are fabricated by forming dielectric films on silicon substrates or semiconductor crystal substrates, followed by integration. Studies have been made to fabricate LSIs having a higher degree of integration and dielectric isolated LSIs relying on SOI technology, by combining semi-conductors with dielectrics. Ferroelectrics which are one class of dielectrics can be applied to non-volatile memories by utilizing their polarization reversal phenomenon. Also, infrared sensors, optical modulators, optical switches, OEIC (opto-electronic integrated circuits) or the like can be fabricated using ferroelectrics. Active research works have been made on the ferroelectric thin film material. It has also been investigated to apply the ferroelectric thin film to recording media of the type wherein polarization reversal by an AFM probe etc. is utilized to record information. In order to establish the non-volatile memories and recording media utilizing the polarization reversal of ferroelectrics, ferroelectric thin film materials having a sufficient residual polarization value to withstand repetitive record and retrieval cycles are necessary.
One of non-volatile memories devised heretofore is a memory of the structure using ferroelectric material in the gate of FET. As described in a technical report issued by the Japanese Electronic Information Communication Society, SDM 93-136, ICD 93-130, (1993-11), page 53, the memory using ferroelectric material in the gate has not reached the practically acceptable level because there remain many outstanding problems associated with their manufacture and the physical properties of ferroelectric thin films. For this type of memory, it is ideal, but difficult to implement a metal-ferroelectric-semiconductor (MFS) structure in the memory cell and therefore, a metal-ferroelectric-insulator-semiconductor (MFIS) structure or metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure must be fabricated. In order that the ferroelectric material undergo polarization reversal to ensure storage operation for this structure, an electric field of sufficient strength must be applied across the ferroelectric material. Since the ferroelectric material and insulator in the MFIS and MFMIS structures become equivalent to a serial connection of capacitors, it is necessary to take appropriate measures for lowering the dielectric constant of ferroelectric material and raising the dielectric constant of insulator in order that a sufficient electric field be applied across the ferroelectric material.
Among ferroelectric thin film materials, investigations have heretofore been made on lead family oxides such as PbTiO3, PZT (PbZrO3-PbTiO3 system), and PLZT (PbZrO3-PbTiO3 system having La2O3 added thereto) and bismuth family oxides such as Bi2Ti2NbO9 for the reason that they exhibit good polarization characteristics.
The PZT and PLZT, however, exhibit a dielectric constant as high as about 1,000 when they are formed into thin film. If they are used as a ferroelectric thin film in the above-mentioned MFIS and MFMIS structures, it is difficult to apply sufficient voltage.
On the other hand, PbTiO3 has a dielectric constant of lower than about 100 at room temperature in bulk form, a spontaneous polarization value of 80 xcexcC/cm2 as theoretically calculated in bulk crystal form which is outstandingly greater than materials of other compositions, and a Curie temperature as high as 500xc2x0 C. Namely, the physical data of PbTiO3 found in the literature are most ideal when considered as the ferroelectric material for memories. Even in thin film form, PbTiO3 has a dielectric constant as low as about 500. Nevertheless, the recent research and development works to form PbTiO3 as a thin film to construct electronic devices revealed many problems. First, the voltage Ec at which polarization reversal occurs is as high as 85 kV/cm. Secondly, crystal defects and semiconductor areas can cause leakage in the thin film. Thirdly, fatigue or repetition properties of polarization reversal are poor. Specifically, the material deteriorates after about 1,000 cycles of polarization reversal.
The aforementioned lead and bismuth family oxides must be crystallized for their thin films to exhibit ferroelectric characteristics. The material can be crystallized by heating at a temperature of higher than 600xc2x0 C. during thin film formation or by annealing a thin film at a temperature of higher than 600xc2x0 C. as disclosed in Jpn. J. Appl. Phys., 31, 3029 (1992), Jpn. J. Appl. Phys., 33, 5244 (1994), and Mat. Res. Soc. Sympo. Proc., 243, 473 (1993). Since lead and bismuth, however, have a high vapor pressure in both elemental and oxide forms, they evaporate during heat treatment at elevated temperatures, incurring a compositional deviation. Composition control is thus difficult.
It is generally desired to use a single crystal form of ferroelectric material in order to ensure optimum device characteristics and reproducibility thereof. Polycrystal-line material is difficult to provide satisfactory device characteristics due to the disturbance of physical quantities by grain boundaries. This is also true for thin film materials, and a ferroelectric epitaxial film which is as close to a complete single crystal as possible is desired. The same applies to ferroelectric thin films for use in non-volatile memories of the above-mentioned MFIS or MFMIS structure, and a dielectric epitaxial film which is as close to a complete single crystal as possible is desired. Also for media (usually of the MFIS or MFMIS structure) wherein information is recorded using an AFM or STM probe, there is a demand for a ferroelectric epitaxial film which is as close to a complete single-crystal as possible because such a film enables to write a high density of bits. In order to form a ferroelectric epitaxial film in the MFIS or MFMIS structure, a metal thin film and a ferroelectric thin film must be epitaxially grown on a silicon substrate which is a semiconductor substrate. No one has succeeded in this epitaxial growth.
Insofar as lead family ferroelectric materials are concerned, a thin film which is free of a compositional deviation and approximate to a single crystal has not been formed on a semiconductor substrate. The high reactivity of lead family ferroelectric materials with silicon serving as the substrate allows for diffusion of Pb into the silicon substrate, which has serious influence on the characteristics of integrated circuits built in the silicon substrate.
An object of the invention is to provide a film structure having a ferroelectric thin film featuring a relatively low dielectric constant, great residual polarization, a low polarization reversing voltage, and minimal deterioration upon repetitive polarization reversal.
Another object of the invention is to provide a process for preparing a lead family ferroelectric thin film of a consistent composition and high crystallinity which could not be formed by conventional processes.
The present invention is directed to a film structure comprising a substrate and a ferroelectric thin film formed on one surface of the substrate.
In a first aspect of the invention, the ferroelectric thin film contains a rare earth element R1, lead, titanium, and oxygen in an atomic ratio in the range:
0.8xe2x89xa6(Pb+R1)/Tixe2x89xa61.3 and
0.5xe2x89xa6Pb/(Pb+R1)xe2x89xa60.99,
has a perovskite type crystal structure, and is of (001) unidirectional orientation or a mixture of (001) orientation and (100) orientation. R1 is at least one rare earth element selected from the group consisting of Pr, Nd, Eu, Tb, Dy, Ho, Yb, Y, Sm, Gd, and Er. Preferably the surface of the substrate on which the ferroelectric thin film is formed has a silicon (100) plane.
In a second aspect of the invention, the surface of the substrate on which the ferroelectric thin film is formed has a silicon (100) plane, and the ferroelectric thin film contains a rare earth element R2, lead, titanium, and oxygen in an atomic ratio in the range:
0.8xe2x89xa6(Pb+R2)/Tixe2x89xa61.3 and
0.5xe2x89xa6Pb/(Pb+R2)xe2x89xa60.99,
has a perovskite type crystal structure, and is of (001) unidirectional orientation or a mixture of (001) orientation and (100) orientation. R2 is at least one rare earth element selected from the group consisting of Pr, Nd, Eu, Tb, Dy, Ho, Yb, Y, Sm, Gd, Er, and La.
In one preferred embodiment, the film structure further includes an insulative subbing thin film between the substrate and the ferroelectric thin film, the insulative subbing thin film having a perovskite type crystal structure and being of (001) unidirectional orientation when it is tetragonal and (100) unidirectional orientation when it is cubic.
In another preferred embodiment, the film structure further includes an intermediate thin film between the substrate and the ferroelectric thin film, the intermediate thin film including a zirconium oxide base thin film which contains as a major component zirconium oxide or zirconium oxide stabilized with a rare earth element inclusive of scandium and yttrium and is of (001) unidirectional orientation when it is tetragonal or monoclinic and (100) unidirectional orientation when it is cubic. The intermediate thin film may further include a rare earth oxide base thin film which is disposed between the zirconium oxide base thin film and the ferroelectric thin film, contains an oxide of a rare earth element inclusive of scandium and yttrium as a major component, and is of (001) unidirectional orientation when it is tetragonal or monoclinic and (100) unidirectional orientation when it is cubic. The film structure may further include an insulative subbing thin film between the intermediate thin film and the ferroelectric thin film, the insulative subbing thin film having a perovskite type crystal structure and being of (001) unidirectional orientation when it is tetragonal and (100) unidirectional orientation when it is cubic.
In a further preferred embodiment, the film structure further includes a conductive subbing thin film disposed close to the ferroelectric thin film, wherein the conductive subbing thin film is a conductive metal thin film constructed of at least one metal selected from the group consisting of Pt, Ir, Os, Re, Pd, Rh, and Ru and/or a conductive oxide thin film constructed of an indiumcontaining oxide or an oxide having a perovskite type crystal structure and is of (001) unidirectional orientation when it is tetragonal and (100) unidirectional orientation when it is cubic.
Preferably, the ferroelectric thin film has a ten point mean roughness Rz of less than 10 nm across a reference length of 500 nm over at least 80% of its surface.
In the ferroelectric thin film, less than 60 atom % of the titanium may be replaced by at least one element selected from the group consisting of Zr, Nb, Ta, Hf, and Ce.
Also contemplated herein are an electronic device comprising the film structure and a recording medium comprising the film structure.
In a further aspect of the invention, there is provided a process for preparing a ferroelectric thin film of oxides containing at least Pb and Ti on a substrate by a multi-source evaporation method, comprising the steps of placing at least lead oxide and TiOx wherein 1xe2x89xa6xc3x97xe2x89xa61.9 as sources in an evaporation chamber and evaporating the sources in the chamber while introducing an oxidizing gas therein.
Preferably the evaporating step is controlled such that the atomic ratio of Pb/Ti elements supplied from the sources represented by E(Pb/Ti) and the atomic ratio of Pb/Ti elements in the ferroelectric thin film represented by F(Pb/Ti) satisfy the relationship: 1.5xe2x89xa6E(Pb/Ti)/F(Pb/Ti)xe2x89xa63.5. In preferred embodiments, the oxidizing gas is a partially radical oxygen; the substrate is heated at a temperature of about 500 to 700xc2x0 C. during the evaporating step; the ferroelectric thin film may further contain at least one element selected from the group consisting of Zr, Nb, Ta, Hf, and Ce.
The process is applicable to the manufacture of the film structures of the first and second aspects.
The composition of the ferroelectric thin film used in the present invention is a PbTiO3 base composition plus a specific rare earth element. As previously mentioned, PbTiO3 has adequate memory properties including spontaneous polarization, dielectric constant, and Curie point, but suffers from the drawbacks of too high voltage Ec required for polarization reversal, leakage in thin film, and fatigue by polarization reversal. The present invention has solved these problems.
By adding a specific proportion of rare earth element to PbTiO3, the invention is successful in lowering the voltage Ec required for polarization reversal and restraining a reduction of residual polarization PR associated therewith. As a result of our precise study on rare earth elements which are unlikely to form semiconductors, we have succeeded in forming a ferroelectric thin film with minimal leakage. Finding that the fatigue by polarization reversal is affected by the type and amount of the rare earth element added, we have succeeded in forming a ferroelectric thin film with improved repetition properties.
More particularly, the ferroelectric thin film according to the invention contains lead (Pb), titanium (Ti), and oxygen (O). It further contains the rare earth element R1 in the first embodiment and the rare earth element R2 in the second embodiment. The second embodiment adds lanthanum (La) as a selection member to the group of rare earth elements which is defined in the first embodiment.
The ferroelectric thin film in the first embodiment is of (001) unidirectional orientation or a mixture of (001) orientation and (100) orientation. Owing to such orientation, the ferroelectric thin film exhibits improved properties, especially minimized leakage.
Japanese Patent Application Kokai (JP-A) No. 94608/1995 discloses to add Sc, Y and lanthanoids to an oxide ferroelectric material containing Pb and Ti as in the first embodiment of the present invention. In Example of this patent publication, a ferroelectric thin film is formed on a silicon (110) substrate by sputtering. The sputtering target used is PbTiO3 containing Y2O3. The ferroelectric thin film formed by such a method in JP-A 94608/1995 does not acquire the orientation defined in the first embodiment of the present invention, but is believed approximate to a polycrystalline film. Poor crystallinity leads to less residual polarization. Example of the patent publication reports the advantage of suppressing the fatigue by polarization reversal, that is, the reduction of residual polarization by repetitive polarization reversal. However, the fatigue by polarization reversal is suppressed in JP-A 94608/1995 for the reason that less stresses are induced by polarization reversal due to the less residual polarization. The suppression of the fatigue by polarization reversal is not regarded practical if the residual polarization is lacking.
JP-A 202039/1995 discloses a lead titanate base ferroelectric material having added thereto Sm and Gd, which are used in the first embodiment of the present invention. In this patent publication, a ferroelectric layer is formed on a stack of a titanium nitride layer and a platinum layer. Sputtering, CVD, sol-gel and laser abrasion methods are described as the candidate method of forming the ferroelectric layer and the sol-gel method is used in Example. Even when the multilayer structure and the formation method described therein are employed, a ferroelectric thin film of specific orientation as defined in the first embodiment of the present invention cannot be formed and, of course, the advantages of the first embodiment are not available.
JP-A 73732/1995 discloses a lead zirconate titanate base ferroelectric material having added thereto Er, which is used in the first embodiment of the present invention. A sol-gel method is described therein as the method of forming the ferroelectric layer. The sol-gel method, however, cannot form a ferroelectric thin film of specific orientation as defined in the first embodiment of the present invention or achieve the advantages of the first embodiment.
In the second embodiment, the substrate on which the ferroelectric thin film is formed is a substrate having a silicon (100) plane as the ferroelectric thin film-bearing surface. It was found in our experiment that a (Pb,La)TiO3 thin film was free of leakage and required a low polarization reversal voltage Ec when it was formed on a silicon (100) substrate and that the leakage and voltage Ec increased when formed on a MgO (100) substrate, for example. For this reason, the second embodiment uses a silicon (100) substrate. The silicon substrate has additional advantages as will be described later.
Ferroelectric thin films of lead titanate and lead zirconate titanate materials having added thereto La, which is used in the second embodiment of the invention, are disclosed in JP-A 138004/1984, 127103/1985, 252005/1987, 252006/1987, and 199745/1992 and JP-B 35249/1991. Some of the ferroelectric thin films described in these patent publications have a mixture of (001) orientation and (100) orientation. All the patent publications use a MgO (100) substrate and none of them form an oriented film on a silicon (100) substrate as in the second embodiment of the present invention. The above-mentioned orientation can be relatively easily accomplished when the MgO (100) substrate is used. However, since MgO has a greater coefficient of thermal expansion than Si, greater stresses are introduced in the ferroelectric thin film when it is cooled to room temperature after deposition. This results in a substantial increase of leakage. The reason why the above-mentioned orientation can be relatively easily accomplished on the MgO (100) substrate is that since the substrate is considerably shrunk, the crystal lattice of the ferroelectric thin film is likely to extend in a direction perpendicular to the substrate surface. As a result, it is apparently observed that c-axis is oriented perpendicular to the substrate surface. In fact, when a ferroelectric thin film of about 300 nm thick was formed on a MgO substrate, this film was found to have substantially poor properties, especially increased leakage, as compared with the degree of orientation estimated from X-ray diffraction. This occurred probably because a great amount of residual stress accumulated in the ferroelectric thin film due to shrinkage of the MgO substrate.
Of the above-referred patent publications, JP-A 199745/1992 discloses that a thin film of La-added lead titanate is formed on a silicon (100) substrate as evidenced by the X-ray diffraction diaphragm of FIG. 2. As also seen from the diaphragm of FIG. 2, only peaks corresponding to (100), (110), and (111) planes are observed in this thin film, which is not of (001) orientation. It is also evident from this patent publication that no one has succeeded in forming a La-added lead titanate thin film of (001) orientation on a silicon (100) substrate.
When thin films are formed from lead family dielectric materials, control of a Pb content is difficult. A compositional deviation is likely to occur since Pb has a higher vapor pressure than the remaining elements. A thin film of lead family ferroelectric material which is free of a compositional deviation and close to a single crystal has never been formed on a semiconductor substrate. Utilizing this nature of Pb in a contrary manner, we have found optimum conditions for multi-source evaporation. Under the optimum conditions, desired ferroelectric crystals are obtained in which just enough Pb is incorporated in perovskite crystals in a self-registering manner. The method of the invention is thus successful in forming a highly crystalline lead family ferroelectric thin film, achieving improved ferroelectric characteristics. Additionally, epitaxial growth becomes possible even on silicon substrates, which is advantageous in applications to electronic devices. Since the composition is based on PbTiO3, the composition has a relatively low dielectric constant so that it is suitable in applications to memories of the structure using ferroelectric as the gate of FET. There is obtained an optimum thin film for memories having the MFIS or MFMIS structure applied thereto.